FIG. 13 shows an engine bench system. In FIG. 13, an engine E/G is combined with a transmission T/M (AT or MT with a clutch) which is connected with dynamometer DY via a coupling shaft. Engine E/G is provided with a throttle actuator ACT for controlling throttle opening.
On the other hand, dynamometer DY is provided with a revolution detector PP and a torque detector (a load cell) LC. The speed or torque of dynamometer DY are controlled on a basis of detection signals of the detectors. This control is implemented by a PID control with a controller (a dynamometer controller) C(s). FIG. 13 shows a case of torque control, where controller C(s) performs a PID-calculation of a deviation between a setting torque input and a measured torque of dynamometer DY at a calculation section, and performs a torque control of dynamometer DY by controlling an output of an inverter INV on a basis of the calculation result.
In the engine bench system that performs the PID control of the dynamometer, the engine may generate a pulsating torque, which may cause the coupling shaft to destroy due to resonance. There is a control system to prevent this resonance destruction, in which the dynamometer is PID-controlled while a resonance point of a mechanical system constituted by the engine, the coupling shaft, and the dynamometer is set to a frequency equal to or lower than the pulsating torque generated by the engine. However, it is very difficult to perform the speed control or shaft torque control with a high-speed response, if the resonance point of the mechanical system is set to be equal to or lower than the engine pulsating torque frequency.
The Applicant has already proposed a shaft torque control system using a μ design method which is one of robust control design theories, which is a stable and high-speed shaft torque control system for an engine bench system where resonance of the shaft is suppressed (for example, refer to patent documents 1 and 2). According to the μ design method, a magnitude of each perturbation of a real system can be expressed in terms of a structural singular value μ. In patent document 1, the magnitude of each perturbation is determined to satisfy robust stability and robust performance conditions during the design of a controller, and is incorporated into the controller to constitute a transfer function of the controller.    Patent Document 1: Japanese Patent No. 3775284.    Patent Document 2: Japanese Patent Application Publication No. 2003-121307.